This invention relates generally to techniques for determining conditions of surfaces, and, more specifically, to doing so by measuring a characteristic of the reflectivity and/or emissivity of surfaces. An example application is the measurement of the thickness of a film on a substrate, such as a film that is formed or removed from a semiconductor wafer during the fabrication of integrated circuits. The thickness measurement is made either simultaneously with the film processing (in situ) or thereafter (in line). More specific applications include in situ measurements of the thickness of a film being reduced or removed by techniques such as wet etching, plasma etching, spin etching or chemical-mechanical-polishing (CMP).
As a result of the development of new semiconductor processing techniques and a steadily shrinking minimum semiconductor element size, a need exists to constantly improve techniques of monitoring and measuring the results of processing, and also to develop new ones. The trend is to make as many measurements of semiconductor wafers as possible in situ, which is usually more difficult to do than as a separate step after the processing. An example of one recent development is described in U.S. Pat. No. 5,769,540, wherein the reflectivity of a surface is measured, from which its emissivity and/or temperature can be determined without contacting the surface. The emissivity measurement is also usable to determine the thickness of a film carried by the substrate. These techniques are particularly useful for making in situ measurements during rapid thermal processing (RTP). Another development, described in U.S. Pat. No. 5,695,660, measures the thickness or level of planarization of both dielectric and metal layers in situ by optical or thermal techniques during etching or CMP, including making the measurements. through the back side of the wafer. When applied to CMP, an optical signal communicates with a wafer being processed through an optical window provided in one of the moving elements such as the wafer carrier. In published international (PCT) application no. WO 97/25660, multiple sensors are carried by a moving component of a CMP machine, with a wireless communication of measurements and control signals provided between the sensors and a host control station. Other patent documents of interest include U.S. Pat. Nos. 5,138,149, 5,190,614, 5,166,525, 5,292,605, 5,308,447, 5,362,969, 5,717,608 and 5,786,886, and PCT publication no. WO93/25893. Each of the foregoing patent publications is from Luxtron Corporation of Santa Clara, Calif., the assignee hereof, and is incorporated herein in its entirety by this reference.
It is a principal object of the present invention to provide further improvements to methods and instruments for optically measuring characteristics of surfaces, such as surfaces of circuit structures partially formed on semiconductor wafers or flat panel displays.
It is a more specific object of the present invention to provide such further improvements to monitor the effects CMP processing.
It is another object of the present invention to provide improved optical methods and instruments for measuring the thickness of layers of dielectric, semiconductor or metal materials carried by a substrate.
It is a further object of the present invention to carry out the foregoing objects simultaneously with processing the surface or layer being monitored (in situ).
It is an even more specific object of the present invention to accurately measure the changing thickness of a layer carried by a substrate, such as a semiconductor wafer, while being processed (in situ) to increase or decrease the layer thickness.
These and additional objects of the present invention are realized by the various aspects of the present invention, which are briefly and generally summarized.
A surface being monitored is illuminated by optical radiation with rays of the incident radiation spread over a wide angle to form a radiation field modified by the surface that is collected over an angle that is as large as practical, and then detected by a sensor, which collection angle may be narrowed, when there is a reason to do so, as the illumination angle becomes very large. The angles of optical radiation illumination and collection are made sufficiently wide so that variations in an optical radiation path and/or of the surface being monitored, other than of the surface optical characteristic of interest, that occur over time or between different copies of the surface are minimized or substantially eliminated in order to improve the accuracy of the resultant measurements. In typical applications, the incident radiation is preferably spread over an angle of at least 45 degrees and up to 180 degrees when striking the surface being monitored, and is also preferably collected over an angle of 45 degrees or more, and up to 180 degrees. However, as the illumination angle is increased, the collection angle can be made narrower. The collected radiation is detected, and the detected radiation is processed to monitor a desired characteristic of the surface. Specific structures of sensors include use of an optical radiation spreading element, such as a diffuser or multi-pass reflector, positioned near to the surface being monitored, and an optical collection element, such as an end of an optical fiber or a lens, is positioned to receive the radiation after being modified by the surface, such as by reflection from the surface. Random or pseudo-random scattering of the illumination radiation is preferred, such as occurs when the optical radiation incident on the surface being monitored has passed through ground glass.
The wide angle illumination and detection significantly reduces the effects of variations in scattering of the optical signals that can occur independently of the quantity desired to be measured. The incident optical radiation, and that modified by the surface being monitored, can be scattered varying amounts that depend upon the surface, angles that the radiation strikes the surface and optical elements, changes over time, and other causes. If the viewing angle is too narrow for a given angular extent of the illumination, for example, any variation in the amount of incident radiation that is scatted into the narrow viewing angle because of differences in scattering properties across the surface being monitored or between different surfaces, versus that which is scattered over wider angles out of view, causes the detected optical signal to vary. Significant variations can also occur when the surface is being viewed through a liquid layer, such as an etchant or slurry, that changes its thickness and other characteristics over time. These factors often cause the measurement signal to have significant amounts of undesired noise. But if the surface is illuminated and viewed over wide angles relative to that through which the incident radiation is scattered by the surface, any liquid etchant on it and by the optical elements, this source of noise is significantly reduced. It is reduced further when the surface is illuminated over wide angles. Illumination and detection over a full hemisphere is ideal but significant improvements are made when lesser angles in the range given above are utilized.
If a surface is illuminated with radiation over a very wide angle, such as 80 degrees or more, the surface modified radiation may be gathered for detection over a small angle, such as 25 or 15 degrees or less, and still provide the advantages described above. A small collection angle is often desired for other purposes, such as to increase the depth of focus of the collection optics, allow practical sized collection optics, increase the sensitivity of the measurement to the thickness of a films being monitored, in those applications, and/or to focus on a very small spot on the surface being monitored.
The term xe2x80x9coptical radiationxe2x80x9d is used in this application to mean electromagnetic radiation in a continuous wavelength range including visible and near visible infrared and ultraviolet wavelengths, generally considered to extend from about 0.2 to about 4 microns. Monitoring the optical radiation modified by a surface usually also includes monitoring the level of radiation incident upon the surface so that the reflectivity or emissivity of the surface, or a related quantity, can be calculated either as the ultimate surface characteristic to be determined or as an intermediate quantity used to calculate some other surface characteristic. By making the measurements in a defined radiation wavelength range and with the geometric constraints discussed above, resulting calculations of the reflectivity or emissivity of the surface are highly meaningful because they are independent of changing conditions unrelated to surface reflectivity and emissivity. The calculation of emissivity is preferred because it is understood in the scientific community to be independent of the geometry, wavelength and other factors that can cause undesired variations in optical measurements.
When a layer of material being formed on or removed from a substrate, in whole or in part, is being monitored, it is preferably illuminated and viewed in the above described manner. An endpoint to the complete removal of a layer is one characteristic of the surface that may be determined. Another characteristic is the thickness of the layer, either in relative or absolute terms. Because surface reflectivity or emissivity is being determined with the effects of varying measurement conditions being minimized, the thickness of a layer can be determined with a high degree of accuracy by a look-up table, model or other relationship between the reflectivity or emissivity and thickness.
When the monitored layer is a metal or other generally opaque material, its reflectivity and emissivity are directly related to the thickness of the layer if the layer is thin enough to be at least partially transparent. For example, when a metal layer is being removed from a substrate, the layer""s reflectivity or emissivity, when measured by the techniques summarized above, is directly related to the thickness of the metal layer once it has become thin enough to be semi-transparent. A metal film on a semiconductor wafer is semi-transparent when its thickness is less than about 500 to 1500 Angstroms, depending on the metal and the wavelength of radiation used. Therefore, for such thin metal layers, their thicknesses can be determined from a look-up table or set of functions that relate the measured reflectivity or emissivity with the layer""s thickness. This makes it possible to measure and control the thicknesses of thin metal films. with precision. Once the metal film is completely removed, then the optical properties being measured are those of the layers under the metal film. The measurement of the reflectivity or emissivity of a metal layer can be conducted with one or more wavelengths of optical radiation.
The thickness of a layer of transparent material, such as a dielectric, can be monitored throughout a range of its entire thickness by viewing a signal resulting from interference of a portion of the incident optical radiation reflected from an outside surface from which material is being removed and another portion of the incident radiation reflected from some other interface in the structure of the substrate and layer. A given table or set of functions are used to relate specific values of the interference signal with layer thicknesses. Interference signals are preferably generated in at least two different wavelengths of optical radiation. Precise measurements of the thickness of a transparent layer are then obtainable without ambiguity under many circumstances when the two or more optical radiation wavelengths and ranges of layer thicknesses are suitable. In other cases, such precision additionally requires knowledge of the approximate layer thickness, as determined by observation or otherwise, in order to obtain a precise measurement of the transparent layer""s thickness without ambiguity.
When two or more wavelengths of radiation are used to illuminate a layer, the radiation can be generated by separate narrow bandwidth radiation sources, such. as light emitting diodes (LEDs), and then the uniquely responsive to one of the source wavelengths. Alternatively, or in combination, the two or more radiation sources are each modulated with a unique frequency. Electrical signals obtained from a single or multiple radiation receiving photodetectors are then filtered to pass the source modulating frequencies in order to obtain a separate signal for each of the illumination wavelengths. The frequencies of modulation are preferably selected to be different from that of any ambient radiation or electrical noise that might be present in the environment in which the measurements are being taken, thereby to significantly reduce adverse effects of any such ambient radiation and noise. The radiation source modulation and photodetector signal filtering technique can also be used when a single optical radiation is directed at the surface being monitored in order to similarly reduce the effects of any ambient radiation and electrical noise that might be present at different frequencies from the modulating frequency.
Although the techniques summarized above are useful to measure a surface characteristic in-line or off-line when processing of the surface is not taking place, it is usually preferable to apply these techniques to in situ monitoring. This saves the extra steps necessary to make the measurements and provides the results in real time. The processing can then be controlled from the measurements, either automatically or by providing the measurements to an operator who then adjusts the processing. One specific processing method where these measurement techniques are particularly useful is CMP, where one or more substrates (such as semiconductor wafers) are held by a carrier, and their exposed surfaces moved across a polishing pad carried by a platen. A slurry of abrasive particles and a chemical etchant is usually also used on the pad. One or more optical radiation sensors, preferably of the wide radiation angle type described above, are installed in either the carrier, if the processing is being viewed from the back side of the substrates, or in the platen, if being viewed from the front side of the substrates. The front side of the wafers can also be periodically viewed in some CMP machines when the wafers are being loaded into or unloaded from the CMP machine, or are periodically moved off the platen within the CMP machine during the CMP process.
When a layer of metal (such as copper, aluminum, tungsten or cobalt) is being removed from a semiconductor surface to leave conductive lines in trenches of an underlying dielectric material layer, the sensitivity to the detection of endpoint by monitoring large areas of the surface is low since the metal remains mainly in the trenches at endpoint. These remaining metal lines can extend over a large percentage of the area of integrated circuit (I.C.) devices being formed on the substrate, so that a reflected signal does not change very much upon reaching endpoint. However, if small areas of the surface in between such I.C. devices are monitored, the sensitivity of endpoint detection increases significantly since there are no metal lines which remain in such areas after endpoint.
An analog signal obtained from a sensor installed in a CMP machine platen is a continuous one, resulting from being scanned across the one or more wafers held by the carrier and the surface of the carrier in between the wafers. If the reflectivity and emissivity of the carrier surface is significantly different than that of the wafers, there is a significant discontinuity of the signal obtained by the photodetector as its view crosses an edge of the wafer. Discrete measurements of the wafer surface are referenced to these discontinuities as a way of identifying the positions of the measurements. In some cases it is desired to take specific measurements near an edge of the wafer outside of patterned circuit die, and in others it is desired to take specific measurements over a patterned location. Control of the location is made possible by use of the edge discontinuity. The measurements at specific positions of the wafers are then monitored over time in order to measure the effect of the processing on the wafer surface.
The representative features of the present invention described above can be implemented alone without use of the others but various combinations of the foregoing summarized features and others are alternatively combinable for use in specific applications. Additional features, advantages and objects of the various aspects of the present invention are included in the following description of various embodiments, which description should be taken in conjunction with the accompanying drawings.